Currently, heat generation density of silica elements keeps on increasing in electronics fields. Moreover, a material having much higher heat dissipation and electric insulation than conventional ones are indispensable in the development of a SiC or GaN element that is expected as a power semiconductor. As a material that satisfies such requirements, there exists a composite material in which a filler having a high thermal conductivity is kneaded into a resin. As the filler for use in such application, magnesium oxide having a higher thermal conductivity than silica and having a lower hardness than alumina has been studied. Magnesium oxide has characteristics of having a high melting point and a high thermal conductivity and being nontoxic and therefore has widely been used as a heat resistant material, filler, or other materials. In recent years, tries to improve the performance of magnesium oxide have also been made by subjecting the surface of magnesium oxide to various kinds of surface treatment. However, according to studies conducted by the present inventors, magnesium oxide has a higher moisture-absorbing property than silica or alumina, and therefore cracks occur due to the volume expansion of the filler attributable to hydration with absorbed water in the use of magnesium oxide as a filler for resin compositions to cause a problem such as lowering of the thermal conductivity. Furthermore, there has also been a problem in imparting a long term stability to semiconductors.
As another thermally conductive material for filler application, a carbon nanotube, boron nitride, aluminum nitride, zinc oxide, or aluminum oxide (alumina) has generally been used. However, these materials have problems as described below. For example, wear in production apparatuses becomes a problem by the use of aluminum oxide because of a high hardness of aluminum oxide. To the contrary, the use of boron nitride having a low hardness causes an effect on the strength required for the materials. With respect to other materials, there is a drawback that the water resistance is poor in aluminum nitride, magnesium oxide, and zinc oxide, and there is a drawback that the electric insulation is poor in carbon nanotubes or other materials.
On the other hand, Patent Literature 1 discloses a magnesium oxide filler for blending into a resin, which gives molded articles excellent in molding processability, thermal conductivity, and water resistance. Moreover, Patent Literature 2 discloses coated magnesium oxide having, on the surface of magnesium oxide, a coating layer containing a composite oxide of magnesium and silicon and/or aluminum. Patent Literature 3 discloses an inorganic filler composite constituted by boehmite or zinc oxide having a granular, horn-like, fibrous, or plate-like shape, the boehmite or zinc oxide bonded or adhered to the surface of a thermally conductive filler such as boron nitride, aluminum nitride, magnesium oxide, a glass bead, or alumina.